A survey for 22 GHz water maser emission from the Arecibo set of OH/IR stars

We report here on our search for water masers from a set of 382 OH/IR stars identified at Arecibo from color selected IRAS sources. Maser emission an integrated flux SI less than or similar to 10(-21) W m(-2) is detected from about half. The detection rate is highest for the bluest stars (>90% for IRAS(25 - 12) < -0.6) and decreases to approximate to 30% for the reddest. We expect multiple observations, better sensitivity and improved positions to raise the eventual detection rate from this sample to about 75%. Some of the undetected OH/IR stars are proto planetary nebulae; most of the rest are stars with very heavy mass-loss rates. The morphological characteristics of our spectra are analyzed: this confirms our expectation that they generally evolve into double-peaked profiles at the higher mass loss rates. Emission in the velocity interval which is blue-shifted with respect to the stellar velocity is statistically stronger than emission from the red-shifted interval. The water outflow velocities reach 75+/-15% of the circumstellar expansion velocities exhibited by OH, while flux variations are rarely larger than a factor of 10.